The invention relates to a method of thermoforming a laminated plastic sheet and to a device for bending thermoplastic laminated plastic sheets.
A method of shaping a laminated plastic sheet is known from the Austrian patent 349 766. In this known method, the region of the sheet to be bent is heated and the sheet is thereupon bent, around a mandrel if necessary, in a bending device. Here, the individual lamellae of the sheet shift relative to one another. Elements serving for shape retention then hold the bent component until the thermoplastic has hardened.
This known method employs a bending toggle which carries a bending plate and is pivoted about a bending mandrel. The bending plate is in constant surface-to-surface contact with the marginal sheet portion to be bent and slides relative to the surface thereof. This sliding motion over a large area is undesired. Since the bending plate lies against the sheet to be bent via an antifriction liner to avoid damage to the surface of the sheet, this bending device cannot hold the bent sheet until the sheet has hardened. Accordingly, a shape retention device is additionally required. The shape retention device includes a pressing die having a concave cylindrical pressing surface which, after the bending procedure, is pressed against the convex surface of the bent sheet portion. This pressing die must be different for each bending radius. Therefore, even the holder of the Austrian patent 349 766 considers the method and device of the patent to be capable of improvement.
A method of this type is known from WO 86/02314. In this known method, the bending device is again in surface-to-surface contact with the marginal sheet portion to be bent throughout at least a large part of the bending procedure. As a result, an undesired shifting of the layers relative to one another occurs at the actual bending location of the sheet to be bent because the bending load is applied far from this actual bending location.
In the device known from the generic WO 86/02314, the pressing member is mounted strictly for up-and-down movement so that only bends of 90 degrees can be obtained, i.e., the method can only be applied to sheets whose margins are to be bent 90 degrees.